It is currently Tue Aug 22, 2017 10:12 am

Instability on a diferential amplifier output

Valves, Transistors, IC's and Components. Bench test techniques, Test Gear - Valve Testers, Scopes, CRT Testers, Bridges, Sig Gens, Pattern Gens, Workshop tours etc
 
Posts: 159
Joined: Fri Mar 09, 2012 6:16 pm
Location: Mayabeque, Cuba

Instability on a diferential amplifier output

Post by Miguel López » Fri May 27, 2016 4:33 pm

Hello

I've been trying a diferential output amplifier, as it is my intention in the future to build a transistor oscilloscope, with better performance that the valve one that I'm currently using. Althought the circuit that I've been testing amplifies the input signal very well, it suffers from instability with no signal applied.

That means that with the input connected to ground, the voltage across points A and B (see picture) is constantly "moving". The range in which it moves, is about 0,6V. Then the trace on the scope will be moving too.

Voltage at points C and D, are very stable, and Vcc (60V) is also very stable. So I can not find tha reason for this instability.

I'm asking for help to solve such instability.

Thanks in advance
Attachments
Diferential Amplifier.jpg

 
Posts: 719
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Instability on a diferential amplifier output

Post by PYE625 » Fri May 27, 2016 6:31 pm

Hello Miguel,

Where did the design originate from, as a matter of interest?
The instability sounds like a very low frequency oscillation and could be caused be a component or pcb layout issue.

 
Posts: 159
Joined: Fri Mar 09, 2012 6:16 pm
Location: Mayabeque, Cuba

Re: Instability on a diferential amplifier output

Post by Miguel López » Fri May 27, 2016 7:37 pm

PYE625 wrote:Where did the design originate from, as a matter of interest?

Me

PYE625 wrote:The instability sounds like a very low frequency oscillation and could be caused be a component or pcb layout issue.

Could you explain a bit more?

Thanks :thumb

 
Posts: 719
Joined: Tue Jan 27, 2015 2:02 pm
Location: Chatteris, Cambs

Re: Instability on a diferential amplifier output

Post by PYE625 » Fri May 27, 2016 8:29 pm

Well done on the design....you are much more clever than me lol !

I was thinking along the lines of feedback due to proximity of components to each other causing stray capacitance.
If not, there may be a need for further decoupling somewhere.

 
Posts: 692
Joined: Sun Aug 07, 2011 8:48 am
Location: Edinburgh

Re: Instability on a diferential amplifier output

Post by peter scott » Fri May 27, 2016 11:09 pm

Hi Miguel,

Your differential stage has quite high gain. Basically 1K/re. It might be interesting to know whether you are seeing a common mode effect i.e. both A and B moving up and down together or whether they are actually moving differentially.

If they are moving together you might need better decoupling of your current source Q3.

You could try reducing the gain a bit by puting a couple of series resistors in the emitters of the differential pair.

Peter

 
Posts: 4203
Joined: Wed May 02, 2012 3:35 pm
Location: Worksop

Re: Instability on a diferential amplifier output

Post by Refugee » Sat May 28, 2016 12:24 pm

I would add a decoupling capacitor on the collector of Q6.

 
Posts: 159
Joined: Fri Mar 09, 2012 6:16 pm
Location: Mayabeque, Cuba

Re: Instability on a diferential amplifier output

Post by Miguel López » Mon May 30, 2016 1:43 pm

peter scott wrote:If they are moving together

They are not. I connect the voltmeter across both collector, and the pointer moves slowly in the range of 0,6 volts. Moving is not periodical, but random.

Refugee wrote:I would add a decoupling capacitor on the collector of Q6.

I can do that. Thanks.

 
Posts: 692
Joined: Sun Aug 07, 2011 8:48 am
Location: Edinburgh

Re: Instability on a diferential amplifier output

Post by peter scott » Mon May 30, 2016 2:56 pm

Hi Miguel,

Re is 25/Ie where Ie is in mA.
If I read your circuit correctly then your tail current source is running 5.88mA so your switching pair have Re of 4.25 ohms.
This means that your switching pair have a gain of 1000/4.25 which is 235. This is quite high when you also have the x10 of the previous stage. (67dB) I suspect you are just seeing the general noise in the earlier parts of the circuit. What is it like ifyou short the input with a capacitor?

Peter

 
Posts: 159
Joined: Fri Mar 09, 2012 6:16 pm
Location: Mayabeque, Cuba

Re: Instability on a diferential amplifier output

Post by Miguel López » Mon May 30, 2016 3:54 pm

I have the input short-circuited, but I can make the test with a capacitor. I have used such gain as I need to get ±12V sweep in the outputs to be able to "move" the trace in the entire screen. I want to measure signal of 10mV, but this is what I did in the simulations. If there is a practical obstacle, I could reduce the gain and measure larger signals only, let's say 100mV.

 
Posts: 692
Joined: Sun Aug 07, 2011 8:48 am
Location: Edinburgh

Re: Instability on a diferential amplifier output

Post by peter scott » Tue May 31, 2016 9:45 am

I think you could still achieve your 10mV sensitivity with reduced gain. Try it with 5 ohms in each emitter lead of the differential pair. If noise is still a problem try shorting the signal with a capacitor to ground at various points along your signal path and it should give you a pointer to the sources of the noise.

Peter


Return to In The Workshop



Who is online

Users browsing this forum: No registered users and 1 guest